Although galvanized steel sheet and zinc alloy-plated steel sheet generally have an excellent corrosion resistance, they are nevertheless subject to a number of problems. One such problem is the nonuniform deposition of the plating metal on the surface of the zinc-plated steel sheet, particularly on galvannealed hot-dip-galvanized steel sheet. This nonuniform deposition promotes cratering during subsequent electrodeposition painting, as a result of an inhomogeneous surface electrical conductivity and/or a nonuniform surface morphology on the steel sheet. In addition, galvanized steel sheet suffers from problems with its workability due to the occurrence of powdering during such working operations as press forming, etc.
The prior art offers the following countermeasures to these problems associated with zinc (alloy) plating.
The method disclosed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number 57-67195 [67,195/82] exploits the superior phosphate conversion treatability and paint film adherence of iron-plated surfaces relative to zinc-plated surfaces. This is achieved in this case by iron-plating (to a prescribed thickness) at least one surface of duplex zinc-plated steel sheet.
Japanese Patent Publication Number 60-37880 [37,880/85] proposes a method for obtaining surface-treated steel sheet which has an excellent secondary adherence for cationic electrodeposition paint films. This is achieved by first iron plating the surface of zinc-plated, zinc composite-plated, or zinc alloy-plated steel sheet and by then executing a thin chromate treatment thereon.
Japanese Patent Application Laid Open Number 59-171645 [171,645/84] teaches a reduction of powdering through the formation of a zinc-rich film (with prescribed proportions of zinc powder and zinc/magnesium alloy powder) over a chromate film on particular types of galvanized steel sheet.
In the method disclosed in Japanese Patent Application Laid Open Number 60-105535 [105,535/85], a chromate film in a prescribed weight and a zinc-rich film layer in a prescribed thickness are overlaid onto Zn/Ni alloy-plated steel sheet. It is reported that powdering is reduced due to the zinc-rich layer.
Japanese Patent Publication Number 56-36868 [36,868/81] discloses a method in which a specified add-on of a nickel plating layer and then a specified add-on of chromate film are formed on zinc-plated steel sheet.
Japanese Patent Publication Number 60-18751 [18,751/85] teaches an improvement in the paint film adherence afforded by a chromate treatment which itself is the subject of a previous patent application. This is achieved by coating the surface of zinc-plated steel sheet with an aqueous solution which contains chromic anhydride, silicic acid colloid, and pyrophosphoric acid. The application of this bath is followed by drying without a water rinse.
The method disclosed in Japanese Patent Application Laid Open Number 61-73900 [73,900/86] proposes the inhibition of cratering in cationic electrodeposition by the formation of a pure zinc-plate film in a prescribed weight on the plated surface of zinc alloy-plated steel sheet.